Glass-base laminates prepared from glass fabric impregnated with an epoxy resin are used in the manufacture of printed circuit boards. The laminate should exhibit, as a general rule, good mechanical, electrical and chemical properties as well as flame retardancy. Nonetheless, these laminates must keep pace with advances in technologies utilizing them which, in turn, lends to more stringent property requirements.
Epoxy glass laminates, having a copper clad on one or both faces, are subject to severe cleaning and etching processes. The laminate, therefore, must possess superior chemical and solvent resistance. Equally important is the peel strength which is the measure of bond strength between the metal foil and laminate. This bond should be sufficiently strong not only as measured at room temperature and for hot peel at 125.degree. C., but also after the solder float test with solder at 500.degree. F. and after continuous service at high temperatures of about 100.degree. C.
In the manufacture of printed circut boards, the laminate is subjected to molten solder by solder floating or solder waving in order to connect the electrical components to the board. If the laminate is defective, it may show numerous cracks on the surface, and in extreme cases blisters may form. This defect is known as "measling", which generally is caused by relatively low heat-resistance or high moisture absorption of the laminates. These adverse properties are affected by the quality of the laminating epoxy resins and, to some extent, by the manufacturing processes.
When electrical components are to be connected to the printed circuit board, a plurality of holes are drilled in the laminate, which may be plated or soldered to provide electrical connections. Holes drilled with high speed carbide tipped drills must be smooth and clean and have good dimensional tolerance so that electronic components can be inserted into the holes and firmly soldered in place. If the epoxy resin composition sets to a very high crosslink density, forming a tight and dense crosslink lattice, the resin may cause rapid wearing of the drills. On the other hand, improperly balanced epoxy resin compositions that have a low heat distortion temperature would give a "soft" laminate, and the heat generated during drilling softens the drill dust which smears the holes or sticks to the drill. In severe cases, the excessive heat generated by drilling might degrade the surrounding resin to show a defect known as "halo" and separation of the metal foil from the laminate around the holes might occur.
Efforts have been made to overcome the aforementioned problems or defects by modifying the epoxy resin formulation or curing system, or by varying the manufacturing operations. While some properties have been improved to the expected levels, it is often discovered that such improvements are at the expense of other properties. For example, epoxy resin having a high functionality, such as an epoxy novolac or tetrakis(hydroxyphenyl)ethane tetraglycidyl ether, may be blended into the diglycidyl ether of bisphenol-A type epoxy resin to improve the elevated temperature performance and high distortion temperature and to reduce the drill smear of the laminate, but the resin results in laminates exhibiting high rigidity, higher drill wear, and possibly low metal peel strength and discoloration. Post curing of laminates made from conventional epoxy resin often improves their properties; it also may cause warpage and surface non-uniformity. Similarly, modifying the curing system may adversely affect metal foil adhesion, color of the laminates, electrical properties, mechanical properties and hardness.
Certain compositions intended to give improved electrical laminates are disclosed in U.S. Pat. Nos. 3,523,037; 4,075,260; 4,311,753 and 4,327,143. For example, the '260 patent discloses blends of two epoxy resins, about 2.5-5 parts by weight of dicyandiamide and a tertiary amine as a catalyst. The tertiary amine is used in 0.3 to 0.8 parts by weight per 100 parts by weight resin solids.
This invention has therefore as its object to provide improved epoxy resin compositions suitable for impregnating glass fabrics for use in the manufacture of laminates, including metal clad laminates, which exhibit superior electrical, physical and chemical properties.